This invention relates to time assignment speech interpolation systems and, more particularly, to a speech detector for such systems.
Because of the extremely high cost of communications transmission facilities, e.g., satellite channels and undersea transmission lines, the prior art has sought various means to maximize the efficiency of existing transmission facilities. One such system is known as a time assignment speech interpolation (TASI) system. In a typical TASI system, calls from n callers are transmitted across, for example n/2 transmission facilities to a remote location. At that location, the n/2 facilities are connected to n output channels. TASI systems operate on the assumption, verified as a statistical fact, that at any given time not all callers will wish to talk simultaneously. In fact, as a general rule, callers are actively talking less than half of the time the talker and the listener are interconnected. Accordingly, TASI systems may be defined as switching systems which interconnect talker and listener only when the talker is actively speaking, provided there is a transmission facility available at that time.
In TASI systems, an input channel is connected to a transmission facility only when a speech signal is detected on that channel by a speech detector. It is desirable that such a speech detector have a variable threshold so that speech may be distinguished from noise even when speech characteristics and ambient noise vary.
In the ideal situation, it is desirable that speech be continuously monitored on each input channel and that the speech detection threshold be frequently updated to reflect changes in the ambient conditions on each input channel. However, in the past, such an arrangement required that each input channel have a separate speech detector associated therewith which greatly increases the cost and complexity of speech detection circuitry.
Accordingly, the prior art has employed centralized speech detectors which are time shared among the various input channels. In this regard, see U.S. Pat. No. 3,520,999--May. The use of centralized speech detection elimintes some duplication of circuit components, however, this benefit is achieved at the expense of the ability to continuously monitor each input channel, since while a time shared speech detector is monitoring one channel other channels are not monitored.
In a centralized speech detector, such as that disclosed by May, each input channel is provided with a level detector which provides a signal proportional to the amplitude of the input signal amplitude on a particular channel at a given time. This signal is then directed to a centralized comparison means where the signal amplitude is compared against a stored threshold for that channel.
It would be desirable to provide a centralized speech detector circuit which includes a centralized amplitude detector as opposed to the distributed level detectors employed by May. More importantly, in the May approach, speech is determined to be present when the amplitude of the input signal exceeds a stored threshold at a particular time. However, at any one time, only one input channel may be monitored. It would be desirable to continuously monitor all input channels in order to provide a speech detector circuit which continuously monitors the signals on all input channels on a centralized basis. Speech is then determined to be present only when the average input signal power over an extended period of time exceeds a stored threshold.
In U.S. Pat. No. 4,028,496 to LaMarche, a speech detection circuit is employed in which the threshold against which the instantaneous input signal sample is compared is the short term running average of the previous samples. However, LaMarche, like May, does not observe the input signal over an extended period of time. It would be desirable to compare the input signal power over a period of time, vis a vis a stored threshold, rather than to compare the instantaneous input signal sample versus a threshold as shown by LaMarche and May and to do so on a centralized basis.